In the production of a metal tube or pipe for a nuclear power plant (hereafter, also referred to as simply a “metal tube or pipe”), in general, after being processed into a metal tube or pipe of predetermined dimensions, the metal tube or pipe is subjected to solid solution heat treatment, and is further subjected to precipitation heat treatment to cause carbides to precipitate along grain boundaries for further improving corrosion resistance, thereby being finished. The precipitation heat treatment is performed with heating to about 700 degrees centigrade and is intended for improving the intergranular corrosion resistance of the metal tube or pipe by causing Cr carbides to precipitate along grain boundaries. The precipitation heat treatment is also intended for removing the residual stress of the metal tube or pipe, which is caused by crookedness straightening and grinding processing to be performed after the solid solution heat treatment.
Since occurrence of discoloration due to the formation of an oxide film on the surface (outer surface or inner surface) of metal tube or pipe in the precipitation heat treatment may lead to corrosion when used as the product, the precipitation heat treatment is generally performed with a vacuum furnace. Since the heat treatment by use of a vacuum furnace is performed by heating the metal tube or pipe held in the furnace in vacuum, radiation heating is often utilized as the heating method. Moreover, the precipitation heat treatment is performed by a batch type treatment in which a plurality of metal tubes or pipes being laid down on a cart or a tray etc. are accommodated in a vacuum furnace and subjected to heat treatment.
FIGS. 1A and 1B are diagrams showing a cart which is used in a batch-type vacuum heat treatment furnace, and on which a metal tube or pipe for a nuclear power plant is laid down, where FIG. 1A is a front view and FIG. 1B is a side view. The cart 2 shown in FIGS. 1A and 1B comprises a carriage 21 with wheels, support pillars 23 fixed to the carriage, and metal cross beams 22 each bridged between two support pillars 23. When the metal tubes or pipes are subjected to precipitation heat treatment with a batch-type vacuum heat treatment furnace by using the cart shown in FIGS. 1A and 1B, the cart is accommodated in the batch-type vacuum heat treatment furnace with a plurality of metal tubes or pipes 1 being laid down on the metal cross beams 22, and the metal tubes or pipes are heated.
A metal tube or pipe for a nuclear power plant has a longer length, since it generally has a diameter of 15 to 22 mm, a wall thickness of 0.9 to 1.3 mm, and a length of 15000 to 27000 mm. Therefore, to prevent the metal tube or pipe from flexing when it is set in place, and from deforming during heat treatment, a plurality of metal cross beams 22 are disposed along a longitudinal direction of the metal tube or pipe in the cart 2, and the cart 2 is assembled by connecting a plurality of carriages 21.
In the precipitation heat treatment, a metal tube or pipe is heated to an elevated temperature to thereby incur thermal expansion in an axial direction of the tube or pipe. On that occasion, the cart or the tray used for laying down the metal tube or pipe also thermally expand, while they have different thermal expansion coefficients from that of the metal tube or pipe. Accordingly, the metal tube or pipe with a longer length inevitably exhibits a large deformation, so the metal tube or pipe and the lay-down portions of the cart or the tray are subjected to rub against each other and thereby scratches are formed on the outer surface of the metal tube or pipe.
The scratches thus formed are made up of a concave portion where the outer surface of the metal tube or pipe is scraped by being rubbed against the lay-down portions, and a convex portion where the scraped metal being a lump and adhered to the outer surface. The concave portion of a scratch may become an initiation of corrosion of the metal tube or pipe, leading to a serious accident in a nuclear power plant. Therefore, the concave portion of a scratch is required to meet the requirement of depth and length limits, compared with those of the standard limit sample, and if the depth or the length of the scratch is more than that of the standard limit sample, it is necessary to polish the outer surface of the metal tube or pipe to take off the concave portion thereof. If such polishing is performed by a machining process, residual stress, which has been once removed from the metal tube or pipe by the precipitation heat treatment, is reproduced, and therefore the polishing is inevitably carried out by hand work.
Further, the convex portion of a scratch may damage a fixture of a steam generator when fixing a metal tube or pipe to the steam generator of a nuclear power plant, thereby leading to a serious accident in the nuclear power plant. For that reason, the convex portion of a scratch should be polished by hand work and be removed as in the case of a concave portion.
Therefore, in the production of a metal tube or pipe, an inspection is made for the presence or absence of a scratch after heat treatment for controlling scratches, and when the scratch is found, the outer surface of the metal tube or pipe is polished by hand work to remove the convex portion thereof. Moreover, when the depth or length of the scratch is more than that of the standard limit sample, the convex portion thereof will be removed by polishing, and therefore the production efficiency deteriorates.
A study has been made to suppress the occurrence of scratches by disposing, without restriction, a thin sheet-like, metallic support member on the metal cross beams 22 of the cart shown in FIGS. 1A and 1B described above so that the support member freely moves in a longitudinal direction of the metal tube or pipe as the metal tube or pipe 1 expands during heat treatment. Furthermore, a study also has been made to prevent the occurrence of scratches by increasing the contact area between the metal tube or pipe and the support member so that the pressure acting on the surface of the contact portion decreases. However, it turned out that those countermeasures are insufficient, and scratches having a depth of about 10 μm are still formed on the outer surface of the metal tube or pipe by precipitation heat treatment.
In the cart shown in FIGS. 1A and 1B described above, under the condition that a heat resistant fabric is disposed on the metal cross beam so that the metal tube or pipe and the metal cross beam are held in indirect contact with each other by virtue of the heat resistant fabric interposed in between, the metal tube or pipe is subjected to precipitation heat treatment in a batch-type vacuum heat treatment furnace. In this way, it is possible to suppress the formation of scratches on the outer surface of the metal tube or pipe. In this case, however, an oxide film may be formed on the outer surface of the metal tube or pipe thereby causing discoloration in the portion where the metal tube or pipe and the metal beam are in indirect contact. If the outer surface of the metal tube or pipe is discolored, corrosion resistance may deteriorate leading to corrosion of the metal tube or pipe. For this reason, it is necessary to remove the discoloration (oxide film) by polishing the outer surface of the metal tube or pipe by hand work.
Up to now, various proposals have been made to suppress the scratches caused by heat treatment and, for example, Patent Literature 1 proposes a heat treatment method using a heat resistant fabric. In Patent Literature 1, it is stated that the formation of scratches on the outer surface of a bar material is suppressed by winding a heat resistant fabric around the outer peripheral surface of a heat treating roller for heating the bar material to be heat treated in a continuous heat treatment in inert gas or reducing gas atmosphere.
Further, Patent Literature 2 relates to a continuous heat treatment method in which a copper tube wound into a coil is laid down on a conveyor to pass through a heating furnace. In Patent Literature 2, when a plurality of copper tubes bundled by a metal band are laid down on a conveyor, a laminated body which is composed of plural stacked sheets of heat resistant fabric and is bound by a heat resistant string or adhesive is used as a cushioning material. It is described that this can suppress the formation of scratches on the surface of the copper tube when it is laid down on the conveyor, and also suppress the metal band from digging into the copper tube at the time of heat treatment.
The heat treatment methods proposed by Patent Literatures 1 and 2 relate to a continuous heat treatment furnace, and are not intended for a vacuum furnace which is used for preventing the discoloration on the surface of materials to be heat treated. Thus, although studies have been made on the scratches caused by heat treatment, no study has been made on the discoloration caused by heat treatment.